1. Field of the Invention
This invention relates to digital switching networks and, more particularly, to a time-space-time switching network that uses a closed-loop link and is capable of switching digitalized voice signals as well as data signals. This switching network lends itself particularly well to use in a computer-controlled private automatic branch exchange (PABX).
2. Prior Art
The recent advances made in printed circuit technology and voice signal digitalization techniques have led to the introduction of time-division switching networks heretofore considered too complex for practical implementation. The basic component of such a network is conventionally comprised of a so-called switching memory into which the contents of incoming time-division channels are sequentially written at addresses corresponding to the numbers allocated to said channels and from which they are read under the control of a memory which sequentially provides the addresses that correspond to the numbers allocated to the outgoing time-division channels. Because a switching memory has but a limited capacity, sets of such memories arranged in matrices are generally employed whenever large volumes of traffic must be dealt with. Arrangements of this type are usually called time-space-time switching networks and are described, for example, in European patent application No. EP-A-39,134. However, the complexity of these networks, while acceptable if the volume of traffic involves more than 15,000 lines, renders their use impractical in the case of a PABX.
British Pat. No. GB-A-1,363,357 describes a time-division switching network organized around a closed-loop link operating in the time-division multiplex mode and closed by a control unit. The control unit establishes thereon time-division communication channels and time-division signaling channels that are seized by subscriber stations directly connected to the loop. However, this network has a limited switching capability and requires the use of intelligent subscriber stations specifically designed for this particular use.